The present invention relates to a core loading strategy, and particularly to a core loading strategy which are suitable for use in a boiling water reactor.
In each light-water type power reactor, e.g., in each boiling water reactor, many fuel assemblies form a lattice in the core, and control rods are vertically moved among the fuel assemblies. The reactor of such type operates in accordance with the excess reactivity of core which is determined by operating the control rods and by the burnable poisons (for example, gadolinia) contained in the fuel assemblies. When the excess reactivity becomes zero, spent fuel assemblies are discharged from the core and the core is charged with new fuel assemblies so that fuel exchange is performed. The arrangement of the fuel assemblies in the core is changed as occasion demands.
In order to increase the core reactivity for improving the economical efficiency of fuel it is preferable that the fuel assemblies having a high degree of reactivity are arranged at the center of the core having a high level of neutron importance. However, in such arrangement, since the output power generated at the radially center of the core is extremely high, the safety margins of fuel rods are reduced. In order to solve these conflicting problems, several methods have been proposed for adjusting the power distribution in the radial direction without producing any loss in reactivity.
Examples of methods for flattening the power distribution in the radial direction of a core include the methods disclosed in U.S. Pat. No. 3,986,924 and Japanese Patent Laid-Open (KOKAI) 53-57388. In these methods, the distribution of infinite multiplication factors in the radial direction of a core is changed by adjusting the amount of uranium-235 in the radial direction of the core. That is, the amount of uranium-235 at the center of the core is smaller than that of uranium-235 in the peripheral portion of the core which surrounds the center thereof so that the infinite multiplication factor at the center of the core is lower than that in the peripheral portion thereof.
There is a method which employs burnable poison for flattening the power distributions in the radial direction and axial direction of the core. This method is disclosed in U.S. Pat. No. 3,799,839. In the radial direction of the core disclosed in U.S. Pat. No. 3,799,839, the amount of burnable poison at the center of the core is greater than the amount of burnable poison at the peripheral portion thereof, and in the axial direction of the core the amount of burnable poison at the center of the core is greater than the amounts of burnable poison at the upper end and lower end portions of the core. In the core disclosed in Japanese Patent Laid-Open (KOKAI) 53-70489, fuel assemblies, each containing a large amount of burnable poison, are disposed at the center of the core, and the other fuel assemblies, each containing a small amount of burnable poison, are disposed in the peripheral portion of the core. In the radial direction of this core, the amount of burnable poison at the cener of the core is greater than the amount of burnable poison at the peripheral portion thereof.
In the core disclosed by U.S. Pat. No. 3,799,839, the concentration of burnable poison of the fuel assemblies disposed at the center of the core is greater than that of the fuel assemblies disposed in the peripheral portion of the core, and the number of the fuel assemblies containing burnable poison and arranged at the center of the core is greater than that of the fuel assemblies containing burnable poison and arranged at the peripheral portion of the core. Japanese Patent Laid-Open No. 57-70489 discloses the concentration of burnable poison and number of fuel assemblies containing burnable poison similarly to U.S. Pat. No. 3,799,839. Japanese Patent Laid-Open No. 57-70489 also discloses an example of a core in which the number of the fuel assemblies containing burnable poison and arranged at the center of the core is equal to that of the fuel assemblies containing burnable poison and arranged at the peripheral portion of the core. Japanese Patent Laid-Open No. 57-70489 discloses that the above-mentioned core can improve the efficiency of fuel utilization without any loss in the shutdown margin. The core disclosed in Japanese Patent Laid-Open No. 57-70489 also can flatten the power distribution in the radial direction of core similarly to the core disclosed in U.S. Pat. No. 3,799,839.